290 Prof. S. F. Baird on the Distribution and 



the following generalizations in regard to the interchange of 

 birds between America and Europe. 



European birds, especially the land-species, reach Greenland 

 and return to the continent by way of Iceland, the Fseroe Islands 

 forming a stepping-stone from Great Britain and Scandinavia. 

 In very rare instances species seem to proceed direct to Green- 

 land, without stopping in Iceland, although this may be due to 

 the fact that, while visiting Iceland, they have not yet been noted 

 there by any naturalist. 



The European birds found on the continent of North America 

 reach it by autumnal movement from Greenland in company 

 with strictly North American species. 



Birds of North America rarely, if ever, reach England from 

 Greenland by direct spontaneous migration by way of Iceland, 

 as shown by the fact that only three of the American birds 

 occurring in Greenland are found in Iceland, and that few of 

 the American species observed in Europe are found in Greenland 

 at all. 



Most specimens of American birds recorded as found in 

 Europe were taken in England (about fifty out of sixty-nine), 

 some of them in Heligoland, very few on the continent (land- 

 birds in only five instances) . 



In nearly all cases these specimens belonged to species abun- 

 dant during summer in New England and the eastern provinces 

 of British America. 



In a great majority of cases the occurrence of American birds 

 in England, Heligoland, and the Bermudas has been in the 

 autumnal months. 



The clue to these peculiarities attending the interchange of 

 species of the two continents will be found in the study of the 

 laws of the winds of the northern hemisphere, as developed by 

 Prof. Henry and Prof. Coffin. These gentlemen have shown 

 (see Prof. Henry's articles on " Meteorology," ^ Report of Com- 

 missioner of Patents for 1856,' p. 489) that " the resultant 

 motion of the surface atmosphere, between latitudes 32° and 58° 

 in North America, is from the west, the belt being twenty 

 degrees wide, and its greatest intensity in the latitude of 45''. 

 This, however, must oscillate north and south at difi'erent sea- 



